Method of producing single-walled carbon nanotubes

ABSTRACT

A method of producing a carbon nanotube product comprising a catalytic particle and carbon nanotubes deposited thereon. The catalytic particles preferably contain Co or Ni metal from Group VIII, and Mo or W metal from Group VIb. The catalytic particle preferably comprises a support material upon which the metals are disposed. The carbon nanotube product is preferably formed by exposing the catalytic particle to a carbon-containing gas at a temperature sufficient to form the carbon nanotubes as a primary portion of a solid carbon product with minor amounts of graphite and amorphous carbon residue.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 11/338,170,filed Jan. 24, 2006, which is a continuation of U.S. Ser. No.10/926,317, filed Aug. 25, 2004, now U.S. Pat. No. 7,094,386, which is acontinuation of U.S. Ser. No. 10/423,687, filed Apr. 25, 2003, now U.S.Pat. No. 6,994,907, which is a continuation of U.S. Ser. No. 09/988,847,filed Nov. 19, 2001, now abandoned, which is a continuation of U.S. Ser.No. 09/389,553, filed Sep. 3, 1999, now U.S. Pat. No. 6,333,016, whichclaims the benefit of U.S. Provisional Application Ser. No. 60/137,206,filed Jun. 2, 1999, each of which are hereby expressly incorporated byreference herein in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to the field of producing carbon nanotubes,and more particularly, but not by way of limitation, to methods ofproducing single-walled carbon nanotubes.

2. Brief Description of the Prior Art

Carbon nanotubes (also referred to as carbon fibrils) are seamless tubesof graphite sheets with full fullerene caps which were first discoveredas multi-layer concentric tubes or multi-walled carbon nanotubes andsubsequently as single-walled carbon nanotubes in the presence oftransition metal catalysts. Carbon nanotubes have shown promisingapplications including nanoscale electronic devices, high strengthmaterials, electron field emission, tips for scanning probe microscopy,and gas storage.

Generally, single-walled carbon nanotubes are preferred overmulti-walled carbon nanotubes for use in these applications because theyhave fewer defects and are therefore stronger and more conductive thanmulti-walled carbon nanotubes of similar diameter. Defects are lesslikely to occur in single-walled carbon nanotubes than in multi-walledcarbon nanotubes because multi-walled carbon nanotubes can surviveoccasional defects by forming bridges between unsaturated carbonvalances, while single-walled carbon nanotubes have no neighboring wallsto compensate for defects.

However, the availability of these new single-walled carbon nanotubes inquantities necessary for practical technology is still problematic.Large scale processes for the production of high quality single-walledcarbon nanotubes are still needed.

Presently, there are three main approaches for synthesis of carbonnanotubes. These include the laser ablation of carbon (Thess, A. et al.,Science 273, 483 (1996)), the electric arc discharge of graphite rod(Journet, C. et al., Nature 388, 756 (1997)), and the chemical vapordeposition of hydrocarbons (Ivanov, V. et al., Chem. Phys. Lett 223, 329(1994); Li A. et al., Science 274, 1701 (1996)). The production ofmulti-walled carbon nanotubes by catalytic hydrocarbon cracking is nowon a commercial scale (U.S. Pat. No. 5,578,543) while the production ofsingle-walled carbon nanotubes is still in a gram scale by laser(Rinzler, A. G. et al., Appl. Phys. A. 67, 29 (1998)) and arc (Haffner,J. H. et al., Chem. Phys. Lett. 296, 195 (1998)) techniques.

Unlike the laser and arc techniques, carbon vapor deposition overtransition metal catalysts tends to create multi-walled carbon nanotubesas a main product instead of single-walled carbon nanotubes. However,there has been some success in producing single-walled carbon nanotubesfrom the catalytic hydrocarbon cracking process. Dai et al. (Dai, H. etal., Chem. Phys. Lett 260, 471 (1996)) demonstrate web-likesingle-walled carbon nanotubes resulting from disproportionation ofcarbon monoxide (CO) with a molybdenum (Mo) catalyst supported onalumina heated to 1200° C. From the reported electron microscope images,the Mo metal obviously attaches to nanotubes at their tips. The reporteddiameter of single-walled carbon nanotubes generally varies from 1 nm to5 nm and seems to be controlled by the Mo particle size. Catalystscontaining iron, cobalt or nickel have been used at temperatures between850° C. to 1200° C. to form multi-walled carbon nanotubes (U.S. Pat. No.4,663,230). Recently, rope-like bundles of single-walled carbonnanotubes were generated from the thermal cracking of benzene with ironcatalyst and sulfur additive at temperatures between 1100-1200° C.(Cheng, H. M. et al., Appl. Phys. Lett. 72, 3282 (1998); Cheng, H. M. etal., Chem. Phys. Lett. 289, 602 (1998)). The synthesized single-walledcarbon nanotubes are roughly aligned in bundles and woven togethersimilarly to those obtained from laser vaporization or electric arcmethod. The use of laser targets comprising one or more Group VI orGroup VIII transition metals to form single-walled carbon nanotubes hasbeen proposed (WO98/39250). The use of metal catalysts comprising ironand at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Moand W), VII (Mn, Tc and Re) or the lanthanides has also been proposed(U.S. Pat. No. 5,707,916). However, methods using these catalysts havenot been shown to produce quantities of nanotubes having a high ratio ofsingle-walled carbon nanotubes to multi-walled carbon nanotubes.

In addition, the separation steps which precede or follow the reactionstep represent the largest portion of the capital and operating costsrequired for production of the carbon nanotubes. Therefore, thepurification of single-walled carbon nanotubes from multi-walled carbonnanotubes and contaminants (i.e., amorphous and graphitic carbon) may besubstantially more time consuming and expensive than the actualproduction of the carbon nanotubes.

Further, one of the greatest limitations in the current technology isthe inability to obtain a simple and direct quantification of thedifferent forms of carbon obtained in a particular synthesis. Currently,transmission electron microscopy (TEM) is the characterization techniquemost widely employed to determine the fraction of single-walled carbonnanotubes present in a particular sample. However, transmission electronmicroscopy can only provide a qualitative description of the type ofcarbon species produced. It is hard to determine how representative ofthe overall production a given transmission electron microscopic imagecan be. Obtaining semi-quantitative determinations of the distributionof different carbon species in a sample with any statisticalsignificance is time consuming, and the method employing transmissionelectron microscopy could not be applied as a routine quality control tolarge-scale operations.

Therefore, new and improved methods of producing nanotubes which enablesynthesis of commercial quantities of substantially pure single-walledcarbon nanotubes and at lower temperatures than previously reported, aswell as methods to directly quantify the different forms of carbonobtained in a particular synthesis, are being sought. It is to suchmethods of producing nanotubes and quantifying synthesis products thatthe present invention is directed.

SUMMARY OF THE INVENTION

According to the present invention, a method for producing carbonnanotubes is provided which avoids the defects and disadvantages of theprior art. Broadly, the method includes contacting, in a reactor cell,metallic catalytic particles with an effective amount of acarbon-containing gas at a temperature sufficient to catalyticallyproduce carbon nanotubes, wherein a substantial portion of the carbonnanotubes are single-walled carbon nanotubes, and the metallic catalyticparticle includes a Group VIII metal, excluding iron, and a Group VIbmetal.

Further, according to the present invention, a method is provided fordetermining catalyst composition and reaction conditions for optimizingproduction of single-walled carbon nanotubes. Broadly, the methodincludes contacting, in a reactor cell, a sample of a product producedby the method for producing carbon nanotubes with an effective amount ofan oxygen-containing gas to oxidize carbon present in the sample whileincreasing the temperature within the reactor cell. The amount of carbondioxide released by the sample is measured, and the specific carbonspecies present in the sample is determined by the release of carbondioxide from the sample at specific temperatures. The catalystcomposition and/or reaction conditions are altered until single-walledcarbon nanotubes are present in substantially higher quantities than allother carbon species in the sample of the product containing nanotubes.

In one aspect of the invention, the metallic catalytic particle is abimetallic catalyst deposited on a support such as silica. The ratio ofthe Group VIII metal to the Group VIb metal in the bimetallic catalystis in the range of from about 1:5 to about 2:1.

An object of the present invention is to provide a method for producingsingle-walled carbon nanotubes in greater quantities and at lowertemperatures.

Another object of the present invention is to provide methods fordetermining quantitatively the different forms of carbon, includingsingle-walled carbon nanotubes, multi-walled carbon nanotubes, andamorphous carbon, present in a sample, and thereby determine theselectivity of a particular catalyst and optimize reaction conditionsfor producing carbon nanotubes.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description when read inconjunction with the accompanying figures and appended claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a transmission electron microscopic image of single-walledcarbon nanotubes from CO disproportionation catalyzed by a Co/Mocatalyst on SiO₂ at 700° C. (100,000 magnification).

FIG. 2 is a transmission electron microscopic image of the sampleemployed in FIG. 1 at higher resolution (400,000 magnification) showingbundles of single-walled carbon nanotubes (SWNTs).

FIG. 3 is a transmission electron microscopic image of the sampleemployed in FIG. 1 showing aligned single-walled carbon nanotubesgrowing in bundles.

FIG. 4 is a transmission electron microscopic image of the sampleemployed in FIG. 1 showing an end view of a single-walled carbonnanotube bundle.

FIG. 5 is a scanning electron microscopic image of the sample employedin FIG. 1 showing a single-walled carbon nanotube bundle growing outfrom the catalytic surface.

FIG. 6 is a Temperature Programmed Oxidation profile of products from COdisproportionation catalyzed by a Co:Mo/SiO₂ catalyst at 700° C.

FIG. 7 is a Temperature Programmed Oxidation profile of products from COdisproportionation catalyzed by a Co catalyst on SiO₂, a Mo catalyst onSiO₂, and a Co:Mo catalyst on SiO₂ at 700° C.

FIG. 8 is a Temperature Programmed Oxidation profile of products from COdisproportionation catalyzed by Co:Mo catalysts on SiO₂ at 700° C. inwhich the molar ratio of Co to Mo is varied.

FIG. 9 is a Temperature Programmed Oxidation profile of products from COdisproportionation catalyzed by Co:Mo/SiO₂ catalyst in which thereaction temperature is varied.

FIG. 10 is a Temperature Programmed Oxidation profile of products fromCO disproportionation catalyzed by Co:Mo/SiO₂ catalyst at 700° C. inwhich the percentage of CO in the carbon-containing gas used in COdisproportionation is varied.

FIG. 11 is a Temperature Programmed Oxidation profile of products fromCO disproportionation catalyzed by Co:Mo/SiO₂ catalyst at 700° C. inwhich the reaction time of CO disproportionation is varied.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method for producing quantitiesof single-walled carbon nanotubes by passing an effective amount of acarbon-containing gas over bimetallic catalytic particles, which consistessentially of a Group VIII and a Group VIb metal, at relatively lowtemperatures; and a method for obtaining a reliable quantitativemeasurement of the yield of single-walled carbon nanotubes present in aproduct containing carbon nanotubes.

Broadly, the method for producing single-walled carbon nanotubescomprises contacting bimetallic catalytic particles consistingessentially of a Group VIII and a Group VIb metal with an effectiveamount of a carbon-containing gas in a reactor heated to a temperatureof from about 500° C. to 1200° C., preferably from about 600° C. toabout 850° C., and more preferably from about 650° to about 750° C. andmost preferably about 700° C. The carbon-containing gas may be suppliedto a reactor in a continuous flow or may be maintained in a stagnantatmosphere.

The phrase “an effective amount of a carbon-containing gas” as usedherein means a gaseous carbon species present in sufficient amounts toresult in deposition of carbon on the metallic catalytic particles atelevated temperatures such as those described hereinbefore, resulting information of carbon nanotubes.

The metallic catalytic particles as described herein include a catalystcomponent. The catalyst as provided and employed in the presentinvention is bimetallic. The bimetallic catalyst contains one metal fromGroup VIII including Co, Ni, Ru, Rh, Pd, Ir, and Pt, and one metal fromGroup VIb including Cr, W and Mo. Specific examples of bimetalliccatalysts which may be employed by the present invention include Co—Cr,Co—W, Co—Mo, Ni—Cr, Ni—W, Ni—Mo, Ru—Cr, Ru—W, Ru—Mo, Rh—Cr, Rh—W, Rh—Mo,Pd—Cr, Pd—W, Pd—Mo, Ir—Cr, Ir—W, Ir—Mo, Pt—Cr, Pt—W, and Pt—Mo.Especially preferred catalysts of the present invention comprise Co—Mo,Co—W, Ni—Mo and Ni—W.

A synergism exists between the two metal components of the bimetalliccatalyst in that metallic catalytic particles containing the bimetalliccatalyst are much more effective catalysts for the production ofsingle-walled carbon nanotubes than metallic catalytic particlescontaining either one or the other metal component as a catalyst. Thissynergistic effect observed with the bimetallic catalyst will bedescribed in more detail hereinafter.

The ratio of the Group VIII metal to the Group VIb metal in the metalliccatalytic particles also affects the selective production ofsingle-walled carbon nanotubes by the method of the present invention.The ratio of the Group VIII metal to the Group VIb metal is preferablyfrom about 1:10 to about 15:1, and more preferably about 1:5 to about2:1. Generally, the concentration of the Group VIb metal (e.g., Mo) willexceed the concentration of the Group VIII metal (e.g. Co) in metalliccatalytic particles employed for the selective production ofsingle-walled carbon nanotubes.

The metallic catalytic particles may comprise more than one metal fromeach of Groups VIII and VIb as long as at least one metal from eachGroup is present. For example, the metallic catalytic particles maycomprise (1) more than one Group VIII metal and a single Group VIbmetal, (2) a single Group VIII metal and more than one Group VIb metal,or (3) more than one Group VIII metal and more than one Group VIb metal.

The bimetallic catalyst may be prepared by simply mixing the two metals.The bimetallic catalyst can also be formed in situ through decompositionof a precursor compound such as bis(cyclopentadienyl) cobalt orbis(cyclopentadienyl) molybdenum chloride.

The catalyst is preferably deposited on a support such as silica (SiO₂),MCM-41 (Mobil Crystalline Material-41), alumina (Al₂O₃), MgO, Mg(Al)O(aluminum-stabilized magnesium oxide), ZrO₂₁ molecular sieve zeolites,or other oxidic supports known in the art.

The metallic catalytic particle, that is, the catalyst deposited on thesupport, may be prepared by evaporating the metal mixtures over flatsubstrates such as quartz, glass, silicon, and oxidized silicon surfacesin a manner well known to persons of ordinary skill in the art.

The total amount of bimetallic catalyst deposited on the support mayvary widely, but is generally in an amount of from about 1% to about 20%of the total weight of the metallic catalytic particle, and morepreferably from about 3% to about 10% by weight of the metalliccatalytic particle.

In an alternative version of the invention the bimetallic catalyst maynot be deposited on a support, in which case the metal componentscomprise substantially 100% of the metallic catalytic particle.

Examples of suitable carbon-containing gases include aliphatichydrocarbons, both saturated and unsaturated, such as methane, ethane,propane, butane, hexane, ethylene and propylene; carbon monoxide;oxygenated hydrocarbons such as acetone, acetylene and methanol;aromatic hydrocarbons such as toluene, benzene and naphthalene; andmixtures of the above, for example carbon monoxide and methane. Use ofacetylene promotes formation of multi-walled carbon nanotubes, while COand methane are preferred feed gases for formation of single-walledcarbon nanotubes.

The carbon-containing gas may optionally be mixed with a diluent gassuch as helium, argon or hydrogen.

In a preferred version of the invention the bimetallic catalyticparticles are disposed within a reactor cell, such as a quartz tube,which is disposed within a furnace or oven, and the carbon-containinggas is passed into the reactor cell. Alternatively, the sample can beheated by microwave radiation. The process may be continuous, whereinthe metallic catalytic particles and carbon-containing gas arecontinuously fed and mixed within the reactor, or the process may be abatch process wherein the carbon-containing gas and metallic catalyticparticles are disposed within the reactor cell and held therein for theduration of the reaction period.

Alternatively, the metallic catalytic particles may be mixed withelectrodes in an arc discharge system to produce single-walled carbonnanotubes and/or multi-walled carbon nanotubes. Alternatively, themetallic catalytic particles may be used in a system exposed to a plasmadischarge induced by microwaves. After the catalytic process has beencompleted, the metallic catalytic particles and the nanotubes areremoved from the reactor. The nanotubes are separated from the metalliccatalytic particles by methods known to those of ordinary skill in theart. Further discussion of such methods is not deemed necessary herein.

The single-walled carbon nanotubes produced herein generally have anexternal diameter of from about 0.7 nm to about 5 nm. Multi-walledcarbon nanotubes produced herein generally have an external diameter offrom about 2 nm to about 50 nm.

The method of obtaining a reliable quantitative measurement of the yieldof single-walled carbon nanotubes is direct and easy to conduct, so thatchanges in selectivity or steady-state production can be readilydetected, facilitating reproducibility and quality control. This methodis based on the Temperature Programmed Oxidation (TPO) technique(Krishnankutty, N. et al. Catalysis Today, 37, 295 (1997)), which iswell known in the art. This technique is frequently used to assess thecrystallinity of carbon and is based on the concept that highlygraphitic materials will be more resistant to oxidation than thosepossessing a short range crystalline order. In the present invention,this technique is adapted to provide a method to determine theselectivity of the production of single-walled carbon nanotubes overmulti-walled carbon nanotubes, as well as the percentages of total solidproduct constituted by each carbon species, including not only single-and multi-walled carbon nanotubes but also amorphous and graphiticcarbon species. Therefore, this method, in combination with the methodfor production of carbon nanotubes as described in detail hereinbefore,will allow for the controlled production of single-walled carbonnanotubes. However, it will be understood that this method can also beused for analysis of any sample containing carbon nanotubes.

Broadly, the method includes passing a continuous flow of a gascontaining oxygen dispersed in a carrier gas, such as 5% oxygen inhelium, over a sample containing carbon nanotubes, such as a catalystcontaining carbon deposits, while the temperature is linearly increasedfrom ambient temperature to about 800° C. The oxygen-containing gas isprovided in an amount effective to oxidize carbon species present in thesample. Oxidation of a carbon species results in the evolution of carbondioxide, and each carbon species, such as single- or multi-walled carbonnanotubes, amorphous carbon, or graphite, is oxidized at a differenttemperature. The evolution of CO₂ produced by the oxidation of eachcarbon species present in the sample is monitored by a massspectrometer. The evolved carbon dioxide is quantified by calibratingwith pulses of known amounts of pure carbon dioxide and oxidation ofknown amounts of graphite, thereby yielding a direct measurement of theamount of carbon which is oxidized at each temperature. That is, eachmol of carbon dioxide detected by the mass spectrometer corresponds toone mol of carbon of the particular species which is oxidized at a giventemperature.

This quantitative method which incorporates the use of TemperatureProgrammed Oxidation, referred to hereinafter as the TemperatureProgrammed Oxidation method, is particularly suitable for thequantitative characterization of single-walled carbon nanotubes becausesingle-walled carbon nanotubes are oxidized in a relatively narrowtemperature range, which lies above the temperature of oxidation ofamorphous carbon and below the temperature of oxidation of multi-walledcarbon nanotubes and graphitic carbon. For instance, the oxidationtemperature of single-walled carbon nanotubes has been shown to be 100°C. higher than that of C₆₀ fullerenes and 100° C. lower than that ofmulti-walled carbon nanotubes by this method. A similar result has beenobtained by the thermo-gravimetric analysis (TGA) method (Rinzler, A. G.et al., Appl. Phys. A, 67, 29 (1998)), confirming the suitability ofthis method for the quantitation of single-walled carbon nanotubes.

The method of Temperature Programmed Oxidation analysis as describedherein can be used to quickly test different catalyst formulations andoperating conditions of nanotube production methods to optimize theproduction of single-walled carbon nanotubes. For example, the optimumbimetallic catalyst present in the metallic catalytic particles, as wellas the optimum molar ratio of the two metals, can be determined byTemperature Programmed Oxidation. Temperature Programmed Oxidation canalso be used to optimize the reaction conditions, such as temperature,time and concentration of carbon in the carbon-containing gas. Forinstance, Temperature Programmed Oxidation results from products run atdifferent reaction temperatures illustrate that the amount of carbondeposited increases as the temperature decreases, but the selectivity toproduce single-walled carbon nanotubes is lower at low temperatures.Therefore, Temperature Programmed Oxidation can be used to find theoptimum reaction temperature for any particular catalyst.

Now it will be understood that although optimization of single-walledcarbon nanotube production has been discussed in detail herein, the samemethod may be used to optimize production of multi-walled carbonnanotubes.

The amount of graphite, amorphous carbon and other carbon residuesformed during the catalytic process are minimized due to the reducedtemperatures that are employed. The amount by weight of graphite oramorphous carbon produced is less than 40% by weight of the total solidmaterial formed during the process, preferably less than 30%, morepreferably less than 20%, and even more preferably less than 10%. Mostpreferably, the amount of graphite, amorphous carbon, and other solidcarbon residue make up less than 5% of the total solid product of thecatalytic process.

The Temperature Programmed Oxidation method as described herein appearsto be the first method described which has the ability to not onlydetermine which carbon species is present in a sample but also determinethe percent of each carbon species present in the sample. This isparticularly helpful in determining what purification steps, if any,should be undertaken before use of the single-walled carbon nanotubes invarious applications. Since the purification steps can be more timeconsuming and expensive than the actual carbon nanotube productionitself, the value of the Temperature Programmed Oxidation method isclearly evident.

The nanotubes produced herein may be used in a variety of applications.For example, they can be used as reinforcements in fiber-reinforcedcomposite structures or hybrid composite structures (i.e. compositescontaining reinforcements such as continuous fibers in addition tonanotubes). The composites may further contain fillers such as carbonblack, silica, and mixtures thereof. Examples of reinforceable matrixmaterials include inorganic and organic polymers, ceramics (e.g.,Portland cement), carbon, and metals (e.g., lead or copper). When thematrix is an organic polymer, it may be a thermoset resin such as epoxy,bismaleimide, polyimide, or polyester resin; a thermoplastic resin; or areaction injection molded resin. The nanotubes can also be used toreinforce continuous fibers. Examples of continuous fibers that can bereinforced or included in hybrid composites are aramid, carbon, glassfibers, and mixtures thereof. The continuous fibers can be woven, knit,crimped, or straight.

The invention will be more fully understood by reference to thefollowing examples. However, the examples are merely intended toillustrate desirable aspects of the invention and are not to beconstrued to limit the scope of the invention.

EXAMPLE 1

Bimetallic catalytic particles containing 10 wt % of mixed cobalt andmolybdenum (1:1 ratio) on a silica substrate were prepared by theincipient wetness impregnation method, in which an appropriate amount ofCobalt Nitrate and Ammonium Heptamolybdate Tetrahydrate were dissolvedtogether in deionized water and gradually dropped on the silica. Ceramicmortar and pestle were utilized to disperse the metals on silica. Theresulting bimetallic catalytic particles were then left to dry atambient conditions for a few hours. The partially dried bimetalliccatalytic particles were then dried in an oven at 80° C. for 12 hours.The dry bimetallic catalytic particles were then calcined in flowing airat 450° C.

For production of nanotubes, 0.1 g of calcined bimetallic catalyticparticles was placed in a vertical quartz tube reactor having an arcinside diameter of 8 mm. The vertical quartz tube reactor containing thecalcined bimetallic catalytic particles was disposed inside a furnacewhich was equipped with a thermocouple and temperature control. Hydrogengas (85 cm³/min) was passed into the reactor from the top of thereactor. The furnace temperature was linearly raised at a rate of 20°C./min from room temperature to 450° C. After 450° C. was reached,hydrogen flow passed into the reactor for an additional 30 min. Thereactor temperature was then increased to 600-700° C. in helium gas.Subsequently, carbon monoxide gas (50% carbon monoxide/50% helium) wasintroduced into the reactor at a flowrate of 100 cm³/min. The contacttime of CO with the calcined bimetallic catalytic particles was variedbetween 15 minutes and 2 hours. After contacting for the prescribedperiod of time, the furnace was turned off and the product was cooleddown in helium to room temperature.

After reaction, the color of the sample had turned to a deep black. Fortransmission electron microscopic analysis of the product, a portion ofthe product was suspended in distilled water by sonication withultra-sound. A few drops of such suspension were deposited on laceycarbon supported on a copper grid. The portion of the product was thendried and inspected in a transmission electron microscope, model JEOLJEM-2000FX at 200 kV. As shown in the transmission electron microscopicimages (FIGS. 1-4), the amount of single-walled carbon nanotubesproduced is clearly seen in large quantities. It is observed that thesesingle-walled carbon nanotubes lay together, roughly aligned as bundles.The transmission electron microscopic images also reveal that thebundles of single-walled carbon nanotubes are coated with amorphouscarbon as from other methods. Most tubes are about 1 nm in diameter,with a few having larger diameters, up to 3.2 nm.

Following transmission electron microscopic analysis, the product wasscanned using a scanning electron microscope, model JEOL JSM-880. Thescanning electron microscopic image represented in FIG. 5 shows thebundles of single-walled carbon nanotubes on the surface of silica.

EXAMPLE 2

Metallic catalytic particles containing the monometallic catalysts ofNi, Co or Mo supported on silica were also prepared by the samemethodology described in Example 1, and their catalytic properties werecompared to that of metallic catalytic particles containing thebimetallic catalyst. After conducting the same treatment in CO at 700°C. as described in Example 1, and doing the same transmission electronmicroscopic analysis, no single-walled carbon nanotubes were observed onthese samples. This result indicates that there is a synergism betweenCo and Mo that makes the combination of two metals, which separatelycannot produce Single-walled carbon nanotubes at this temperature, avery effective formulation.

EXAMPLE 3

A series of metallic catalytic particles containing 6 wt % Co—Mobimetallic catalysts were prepared on different supports (Si0₂, MCM-41,Al₂0₃, Mg(Al)0, and Zr0₂), and their nanotube production abilities werecompared, following the same CO disproportionation methodology asemployed in Example 1. Table 1 summarizes the results of theseexperiments.

TABLE I Effect of Catalyst Support on Carbon Deposit Morphology CatalystObserved Morphology of Carbon Deposits Co:Mo/SiO₂ major amount ofsingle-walled carbon nanotubes, minor amounts of multi-walled carbonnanotubes and graphite Co:Mo/MCM-41 major amount of single-walled carbonnanotubes, minor amounts of multi-walled carbon nanotubes and graphiteCo:Mo/Al₂O₃ minor amounts of single- and multi-walled carbon nanotubesand graphite Co:Mo/Mg(Al)O minor amount of graphite, small amount ofsingle-walled carbon nanotubes Co:Mo/ZrO₂ minor amount of graphite,small amount of single-walled carbon nanotubes

EXAMPLE 4

Following the same procedure as that in Example 1, it was observed thatmetallic catalytic particles containing a Co—W bimetallic catalystdeposited on Si0₂ with a Co/W molar ratio of 1.0 gave similar productionof single-walled carbon nanotubes as that of the Co—Mo/Si0₂ metalliccatalytic particles. As in the case of the Co—Mo series, it was observedthat metallic catalytic particles containing only W/Si0₂ without Co didnot form single-walled carbon nanotubes.

EXAMPLE 5

Carbon species produced by using metallic catalytic particles containinga 6 wt % Co—Mo bimetallic catalyst (1:2 ratio) supported on silica bythe same CO disproportion methodology as described in Example 1 wereanalyzed by the Temperature Programmed Oxidation method, as shown inFIG. 6.

For Temperature Programmed Oxidation analysis, 50 mg of sample obtainedfrom the product of CO treatment at 700° C. was placed in a quartz tubereactor similar to that employed in Example 1. A continuous flow of 5%oxygen/95% helium was passed into the reactor, and the temperature ofthe furnace was increased from ambient temperature to 800° C. at a rateof 11° C. per minute, and then held at 800° C. for 1 hour. CO₂ evolutionwas measured by mass spectrometry to determine the amount of carbonspecies oxidized at each temperature.

Mass spectrometry measures the partial pressure of CO₂ in the quartztube, which gives an arbitrary value. This value was then normalized bysubtracting the background level, which was calculated followingcalibration with 100 ml pulses of CO₂ and oxidation of known amounts ofgraphite. The adjusted value was directly proportional to the mol CO₂oxidized at a particular temperature, which is directly proportional tothe mol of a particular carbon species which is present in the sample.From these values, the percentage of the total solid product of thecatalytic process represented by single-walled carbon nanotubes can becalculated.

The Temperature Programmed Oxidation profile of the carbon speciesproduced on the Co:Mo/SiO₂ metallic catalytic particles (labeled “Co:Mo1:2”) presented a small oxidation peak centered at about 330° C., whichis ascribed to the oxidation of amorphous carbon, and a major peakcentered at about 510° C., which is marked in the figure with an arrowand ascribed to the oxidation of single-walled carbon nanotubes.

Two reference samples were also investigated by the TemperatureProgrammed Oxidation method and their profiles included in FIG. 6. Thefirst reference (labeled “Graphite”) was a graphite powder physicallymixed with the Co:Mo/SiO₂ metallic catalytic particles. The oxidation ofthis form of carbon occurred at very high temperatures, starting atabout 700° C., and completed after holding 30 minutes at 800° C.

The second reference sample was a commercial sample of purifiedsingle-walled carbon nanotubes, obtained from Tubes@Rice (RiceUniversity, Houston, Tex.). This sample was provided in a liquidsuspension of 5.9 grams/liter, containing a non-ionic surfactant TritonX-100. For Temperature Programmed Oxidation analysis, the Co:Mo/SiO₂metallic catalytic particles were impregnated with the single-walledcarbon nanotube suspension in a liquid/catalyst ratio of 1:1 by weight,in order to obtain approximately 0.6 wt % single-walled carbon nanotubeson the sample. The Temperature Programmed Oxidation profile of thisimpregnated sample (labeled “Tubes@Rice”) exhibited two peaks, a lowtemperature peak that corresponds to the oxidation of the surfactant,and a second peak at about 510° C., which corresponds exactly to theposition ascribed to the oxidation of single-walled carbon nanotubes. Todetermine that the first peak was indeed due to the oxidation of thesurfactant, an identical sample with a blank solution containing onlythe surfactant in the same concentration was prepared. The TemperatureProgrammed Oxidation profile (labeled “Blank solution”) matched thefirst peak of the “Tubes@Rice” profile, demonstrating that indeed thispeak corresponds to the surfactant Triton.

The quantification of the amount of single-walled carbon nanotubes inthe “Tubes@Rice” reference sample from the CO₂ produced by theTemperature Programmed Oxidation method gave a value of 0.64 wt %, whichis in good agreement with the amount of single-walled carbon nanotubesloaded in the sample (0.6 wt %). This result demonstrates that theTemperature Programmed Oxidation method of the present invention can beused to directly quantify the percentage of a particular carbon species,such as single-walled carbon nanotubes, multi-walled carbon nanotubes,and amorphous carbon, present in a product obtained by the nanotubeproduction method. Currently, no other method of directly quantifyingthe fraction of a total solid product of nanotube production representedby a particular carbon species exists.

EXAMPLE 6

Temperature Programmed Oxidation profiles of the products from COdisproportionation catalyzed by metallic catalytic particles containingthe monometallic catalysts of Co or Mo supported on silica weregenerated by the method employed in Example 5 and were compared to theTemperature Programmed Oxidation profile of products from COdisproportionation catalyzed by the bimetallic catalyst. The TemperatureProgrammed Oxidation method clearly demonstrates the synergistic effectexhibited by Co and Mo, which was also observed by transmission electronmicroscopy as described in Example 2.

As shown in FIG. 7, the Temperature Programmed Oxidation profile of thesample containing Mo/SiO₂ metallic catalytic particles (labeled “Mo”)indicates that Mo alone does not produce carbon nanotubes; the “Mo”Temperature Programmed Oxidation profile only contains a smalllow-temperature peak corresponding to amorphous carbon. Similarly, theTemperature Programmed Oxidation profile of the sample containingCo/SiO₂ metallic catalytic particles (labeled “Co”) indicates that Coalone is not selective for the production of single-walled carbonnanotubes and generates mainly graphitic carbon and multi-walled carbonnanotubes, which, as described above, are oxidized at highertemperatures than single-walled carbon nanotubes. By contrast, thecombination of the two metals results in high selectivity forsingle-walled carbon nanotubes, and the sample containing Co:Mo/SiO₂metallic catalytic particles (labeled “Co:Mo=1:2”, wherein the Co:Moratio was 1:2), exhibits a large peak centered at about 510° C. and isascribed to single-walled carbon nanotubes. Because no other peaks areevident, it can be assumed that single-walled carbon nanotubes areprovided as a large percentage of the total solid product of nanotubeproduction.

The percentages of single-walled carbon nanotubes, amorphous carbon, andmulti-walled carbon nanotubes and graphite present in the catalyticproducts are listed in Table II.

TABLE II Synergistic Effect Exhibited by Co and Mo Single- Multi-WalledWalled Carbon Amorphous Carbon Nanotubes and Catalyst Carbon % Nanotubes% Graphite % Co 38 11 51 Mo 95 5 0 Co:Mo (1:2) 8 88 4

EXAMPLE 7

Temperature Programmed Oxidation profiles of the products from COdisproportionation catalyzed by metallic catalytic particles containingCo:Mo bimetallic catalysts at Co:Mo ratios of 1:4, 1:2, 1:1 and 2:1 werecompared to determine the effect of varying the Co:Mo molar ratio in theCo:Mo/SiO₂ metallic catalytic particles. The Temperature ProgrammedOxidation profiles were generated by the same methodology as describedin Example 5. As shown in FIG. 8, the Co:Mo/SiO₂ metallic catalyticparticles containing Co:Mo molar ratios of 1:2 and 1:4 exhibited thehighest selectivities towards single-walled carbon nanotubes. The arrowindicates the center of the peak corresponding to the oxidation ofsingle-walled carbon nanotubes. The Temperature Programmed Oxidationprofile of these samples indicate that these catalysts produced mostlysingle-walled carbon nanotubes, with a small amount of amorphous carbon.An increase in the Co:Mo ratio did not enhance the production ofsingle-walled carbon nanotubes, but it did accelerate the formation ofmulti-walled carbon nanotubes and graphitic carbon, as shown by theincreasing size of the peaks in the region of about 600° C. to about700° C. of the Temperature Programmed Oxidation profile labeled“Co:Mo=2:1”.

From the Temperature Programmed Oxidation profiles of FIG. 8,selectivity values for each of the catalysts were estimated, and arelisted in Table III.

TABLE III Effect of Co:Mo Molar Ratio on Production of Single- walledCarbon Nanotubes Multi-Walled Single-Walled Carbon Co:Mo CatalystAmorphous Carbon Nanotubes and Molar Ratio Carbon % Nanotubes % Graphite% 2:1 12 57 31 1:1 16 80 4 1:2 8 88 4 1:4 5 94 1

EXAMPLE 8

FIGS. 9-11 demonstrate the use of the Temperature Programmed Oxidationtechnique to optimize reaction conditions. CO disproportionation wascatalyzed by Co:Mo/SiO₂ metallic catalytic particles (1:1 molar ratio),and the methodology used was similar to that described in Example 1,with the exceptions that in FIG. 9 the reaction temperature varied, inFIG. 10 the concentration of CO varied, and in FIG. 11 the reaction timevaried. The products of CO disproportionation were analyzed by theTemperature Programmed Oxidation method described in Example 5.

In FIG. 9, Temperature Programmed Oxidation profiles of carbon speciesproduced when the temperature of the reactor was 600° C., 700° C. and800° C. are shown. These profiles demonstrate that the amount of carbondeposited increases as the temperature decreases; however, theselectivity to single-walled carbon nanotubes is lower at lowertemperatures. The Temperature Programmed Oxidation technique can be usedto identify the optimum reaction temperature for any particularcatalyst, and in this case, the optimum temperature is 700° C. Thepercentages of the catalytic products represented by single-walledcarbon nanotubes, amorphous carbon, and multi-walled carbon nanotubesand graphite are listed in Table IV.

TABLE IV Effect of Reaction Temperature on Production of Single- WalledCarbon Nanotubes Single- Multi-Walled Walled Carbon Amorphous CarbonNanotubes and Temperature Carbon % Nanotubes % Graphite % 600° C. 16 5529 700° C. 16 80 4 800° C. 25 61 14

In FIG. 10, Temperature Programmed Oxidation profiles of carbon speciesproduced when the concentration of CO in the carbon-containing gas is1%, 20%, 35% and 50% are shown. These profiles indicate that the amountof single-walled carbon nanotubes produced is a strong function of theconcentration of CO in the carbon-containing gas.

In FIG. 11, Temperature Programmed Oxidation profiles of carbon speciesproduced when the reaction time was 3 minutes, 10 minutes and 1 hour areshown. The reaction time refers to the time in which the reactor washeld at 700° C. and the CO was in contact with the metallic catalyticparticles. These Temperature Programmed Oxidation profiles demonstratethat the yield of single-walled carbon nanotubes significantly increaseswith time during the first 10 minutes, but the growth is much lesspronounced beyond that time.

Now it will be understood that the Temperature Programmed Oxidationmethod is a catalytic process in which the metals present in the samplecatalyze the oxidation of the carbon species. Therefore, if the natureof the catalyst is significantly changed, the position of the oxidationpeaks may appear shifted from the peaks described in the previousexamples, even though the carbon structures represented by the peaks arethe same. For example, it has been observed that modification of thecatalyst support may result in such shifts. Therefore, for each catalystused in the methods of the present invention, a complete TemperatureProgrammed Oxidation analysis of the catalyst as well as operatingconditions should be performed with the appropriate references toidentify peak shifts as well as optimum operating conditions.

Changes may be made in the construction and the operation of the variouscomponents, elements and assemblies described herein or in the steps orthe sequence of steps of the methods described herein without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A method for producing carbon nanotubes, comprising: providingcatalytic particles comprising at least one Group VIII metal selectedfrom Co and Ni, and at least one Group VIII metal selected from Mo andW, the metals disposed upon a support material selected from the groupconsisting of silica, silicon, MCM-41, alumina, MgO, aluminum-stabilizedmagnesium oxide, ZrO₂, molecular sieve zeolites, oxidic supports,quartz, glass, and oxidized silicon surfaces; providing acarbon-containing gas; and feeding the catalytic particles and thecarbon-containing gas into a reactor wherein the catalytic particles andcarbon-containing gas are mixed at a temperature sufficient tocatalytically produce a solid carbon product primarily comprising carbonnanotubes wherein graphite and amorphous carbon comprise less than 10%of the solid carbon product.
 2. The method of claim 1 wherein the GroupVIII metal is Co and the Group VIII metal is Mo.
 3. The method of claim1 wherein the Group VIII metal is Ni and the Group VIII metal is Mo. 4.The method of claim 1 wherein the Group VIII metal is Co and the GroupVIb metal is W.
 5. The method of claim 1 wherein the Group VIII metal isNi and the Group VIb metal is W.
 6. The method of claim 1 wherein theratio of the Group VIII metal: Group VIb metal is about 1:20 to about15:1.
 7. The method of claim 1 wherein the carbon-containing gas isselected from the group consisting of saturated hydrocarbons, aliphatichydrocarbons, oxygenated hydrocarbons, alcohols, aromatic hydrocarbons,carbon monoxide, and mixtures thereof.
 8. The method of claim 1 whereingraphite and amorphous carbon comprise less than 5% of the solid carbonproduct.
 9. The method of claim 1 wherein solid carbon product comprisescarbon nanotubes having diameters of 2 nm to 50 nm.
 10. The method ofclaim 1 wherein the support material is silica.
 11. The method of claim1 wherein the support material is MgO.
 12. The method of claim 1 whereinthe support material is alumina.
 13. The method of claim 1 wherein thetemperature is from 500° C. to 1,200° C.